1. Field of the Invention
The present invention relates to a method of making a brazeable metal pipe made of a metal such as aluminum(including its alloys), and more particularly relates to a method of making a brazeable metal pipe having tube-insertion apertures formed with guide lugs, wherein the metal pipe is adapted for use as headers constituting heat exchangers such as a condenser, a radiator and an evaporator which are manufactured by the brazing method.
2. Description of the Prior Art
FIGS. 2A and 2B show an example of the conventional heat exchangers of the so-called parallel flow type, which comprises a pair of hollow headers 3 and 4 disposed in parallel with one another. This heat exchanger further comprises a plurality of flat tubes 1 having a depressed circular shape in cross section, with the flat tubes being also arranged in parallel with one another. Each flat tube has its both ends connected to the headers in fluid communication therewith. The heat exchanger still further comprises fins serving as heat release means, and each fin is interposed between the adjacent flat tubes.
It has been a general practice in making the headers 3 and 4 of the prior art heat exchangers that a brazing sheet, which comprises a core sheet having one or both surfaces thereof clad with a brazing substance layer, is rolled into a cylindrical shape. The apertures, which are of an elliptical shape identical with the cross-sectional shape of the flat tubes, are formed through the pipe's wall for receiving said tubes so as to form a row extending longitudinally of the pipe. Thus, the flat tubes 1 are inserted in the apertures of the headers 3 and 4 and brazed thereto so that they are secured one to another.
It will be noted here that the headers have been manufactured by punching such apertures through a seam-welded pipe made of the brazing sheet. Therefore, the pipe has been likely to be collapsed or deformed during the punching process.
In order to resolve this problem, the present Applicant proposed an improved method of making a brazeable pipe, wherein a raw strip composed of a brazing sheet is bent at first transversely or longitudinally to form a convex bulged portion. The bulged portion is then punched to provide the tube insertion apertures, before opposite sides of the raw strip are caused to abut each other and brazed one to another into a cylindrical shape (see Japanese Patent Publication Hei. 3-18982).
According to this improved method, an anvil of a shape similar to the bulged portion is fitted thereon when the apertures are punched. Thus, the previously proposed method is advantageous in that the pipe wall is well protected from being collapsed or deformed, thereby giving high precision pipes.
This method of making the brazeable metal pipe may be most effective if the tube insertion apertures are of an elliptical or other simple shape. However, if said apertures are not of such a simple shape but of a considerably sophisticated shape as is demanded recently, then certain new problems will take place as will be detailed below.
As shown in FIG. 1, guide lugs 7 are formed integral with at least one of edges defining therebetween the minor axis of an ellipse, i.e., the tube-insertion aperture 5 of the header. Those guide lugs 7 extend in parallel with the flat tube 1 so that each tube is readily and smoothly inserted into the aperture, and is held stable in place after inserted. Due to such guide lugs, the inserted tube has an increased surface area in contact with the periphery of the aperture, which is helpful both for support and brazing connection purposes.
It may be regarded as feasible to form each tube-insertion aperture with the guide lug 7 in such a manner that a raw or unfinished aperture of an elliptical shape is punched at first, wherein the raw aperture is surrounded by opposite edges between which the minor axis of the ellipse is defined, with the edges extending in opposite directions to provide protrusions which will become the guide lugs at the succeeding step when bent to take their position in the direction of the thickness of this raw strip.
It is however a new problem that a punching die of a sophisticated pattern need be used to form in one-shot operation the raw apertures through the raw strip which has the bulged portion previously bent into an arc-shaped contour. In detail, the die must have a cutter blade of such a three-dimensional configuration that is in a good alignment not only with the aperture in its plan view, but also with the arc-shaped contour in its side-elevational view. Such a complicated die cannot be manufactured at a low reasonable cost, and its cutter blade tends to be damaged or worn away in a short time.